.ON APRIL 2ND a device resembling a snowfall device on the deck of the USS Hornet, an obsolete aircraft-carrier anchored in San Francisco Gulf, started creating a haze of salty spray particles. The scientific collaboration behind the project, the Marine Cloud Brightening (MCB) programme, led by the College of Washington, is actually exploring whether such aerosols may help to reduce weather improvement with a type of "geoengineering". What does this phrase way-- as well as why is it therefore controversial?